my 99 passat turbo check engine light is on.what could it be?

 

Could be hundred's of things. You are going to have to buy a DTC scanner or take it to a mechanic. I don't know if you have a PEPBoys where you're from but I think they will do it for free. Be warned though they work on commission and they tried to pull the wool over my eyes once. I'd ask to see the code.